Graphene-Reinforced Polymer Matrix Composites

ABSTRACT

A graphene-reinforced polymer matrix composite comprising an essentially uniform distribution in a thermoplastic polymer of about 10% to about 50% of total composite weight of particles selected from graphite microp articles, single-layer graphene nanoparticles, multilayer graphene nanoparticles, and combinations thereof, where at least 50 wt % of the particles consist of single- and/or multi-layer graphene nanoparticles less than 50 nanometers thick along a c-axis direction. The graphene-reinforced polymer matrix is prepared by a method comprising (a) distributing graphite microparticles into a molten thermoplastic polymer phase comprising one or more matrix polymers; and (b) applying a succession of shear strain events to the molten polymer phase so that the matrix polymers exfoliate the graphite successively with each event until at least 50% of the graphite is exfoliated to form a distribution in the molten polymer phase of single- and multi-layer graphene nanoparticles less than 50 nanometers thick along a c-axis direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. ProvisionalApplication No. 62/030,799, filed on Jul. 30, 2014, the entiredisclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to graphene-reinforced polymer matrixcomposites prepared by high efficiency mixing methods to transformpolymer composites containing well-crystallized graphite particles intonano-dispersed single- or multi-layer graphene particles, the compositeshaving various commercial applications.

BACKGROUND OF THE INVENTION

Polymer compositions are being increasingly used in a wide range ofareas that have traditionally employed the use of other materials, suchas metals. Polymers possess a number of desirable physical properties,are light weight, and inexpensive. In addition, many polymer materialsmay be formed into a number of various shapes and forms and exhibitsignificant flexibility in the forms that they assume, and may be usedas coatings, dispersions, extrusion and molding resins, pastes, powders,and the like.

There are various applications for which it would be desirable to usepolymer compositions, which require materials with strength propertiesequivalent to metals. However, a significant number of polymericmaterials fail to be intrinsically strong enough for many of theseapplications.

Graphene is a substance composed of pure carbon in which atoms arepositioned in a hexagonal pattern in a densely packed one-atom thicksheet. This structure is the basis for understanding the properties ofmany carbon-based materials, including graphite, large fullerenes,nano-tubes, and the like (e.g., carbon nano-tubes are generally thoughtof as graphene sheets rolled up into nanometer-sized cylinders).Graphene is a single planar sheet of sp²bonded carbon atoms. Graphene isnot an allotrope of carbon because the sheet is of finite size and otherelements can be attached at the edge in non-vanishing stoichiometricratios.

When used to reinforce polymers, graphene in any form increases polymertoughness by inhibiting crack propagation. Graphene can also be added topolymers and other compositions to provide electrical and thermalconductivity. The thermal conductivity of graphene makes it an idealadditive for thermal management (e.g., planar heat dissipation) forelectronic devices and lasers. Some commercial applications of carbonfiber-reinforced polymer matrix composites (CF-PMCs) include aircraftand aerospace systems, automotive systems and vehicles, electronics,government defense/security, pressure vessels, and reactor chambers,among others.

Progress remains very slow in the development of low-cost methods toeffectively produce graphene-reinforced polymer matrix composites(G-PMCs). Currently, some of the challenges that exist affecting thedevelopment of G-PMCs viable for use in real-world applications includethe high cost of the materials and the impracticality of the presentlyused chemical and/or mechanical manipulations for large-scale commercialproduction. It would thus be desirable for a low-cost method to producea G-PMC suitable for large-scale commercial production that offers manyproperty advantages, including increased specific stiffness andstrength, enhanced electrical/thermal conductivity, and retention ofoptical transparency.

SUMMARY OF THE INVENTION

The present disclosure provides a graphene-reinforced polymer matrixcomposite (G-PMC) prepared by polymer processing methods comprising insitu exfoliation of well-crystallized graphite particles dispersed in amolten thermoplastic polymer matrix. Extrusion of a graphite-polymermixture shears the graphite to exfoliate graphene sheets and improvesthe mechanical properties of the bulk polymer.

One aspect of the invention is directed to a graphene-reinforced polymermatrix composite comprising an essentially uniform distribution in athermoplastic polymer matrix of between about 10 wt % and about 50 wt %,preferably about 20 wt % to about 40 wt %, more preferably about 25 wt %to about 35 wt %, and most preferably about 30 to about 35 wt % of totalcomposite weight of particles selected from the group consisting ofgraphite microparticles, single-layer graphene nanoparticles,multi-layer graphene nanoparticles, and combinations of two or morethereof where at least 50 wt % of the particles consist of single-and/or multi-layer graphene nanoparticles less than 50 nanometers thickalong a c-axis direction; and the thermoplastic polymer is selected fromthe group consisting of polyamides, ABS polymers, polyacrylonitriles,polylactic acids, polyglycolic acids, and mixtures of two or morethereof. In one embodiment the thermoplastic polymer comprises apolyamide. In a preferred embodiment the polyamide is selected from thegroup consisting of aliphatic polyamides, semi-aromatic polyamides,aromatic polyamides and combinations of two or more thereof. In a onepreferred embodiment the polyamide is an aliphatic polyamide selectedfrom the group consisting of polyamide-6,6; polyamide-6,9;polyamide-6,10; polyamide-6,12; polyamide-4,6; polyamide-6 (nylon-6);polyamide-11 (nylon-11); polyamide-12 (nylon-12) and other nylons. In aparticularly preferred embodiment the aliphatic polyamide ispolyamide-6,6.

In another embodiment of the above graphene-reinforced polymer matrixcomposite, the polyamide is a semi-aromatic polyamide. In yet anotherembodiment, the polyamide is an aromatic polyamide, also known as anaramid.

The graphene-reinforced polymer matrix composites can further compriseadditional components which provide desirable properties to the finalcomposite. In one embodiment the graphite may be doped with otherelements to modify a surface chemistry of the exfoliated graphenenano-particles. A surface chemistry or nanostructure of the dispersedgraphite may be modified to bond with the polymer matrix to increasestrength and stiffness of the graphene-reinforced composite. In oneembodiment, directional alignment of the graphene nanoparticles is usedto obtain one-, two- or three-dimensional reinforcement of the polymermatrix phase. In one embodiment the polymer chains are inter-molecularlycross-linked by single- or multi-layer graphene sheets having carbonatoms with reactive bonding sites on the edges of said sheets. Inanother aspect of the invention, the above graphene-reinforced polymermatrix composite further comprises at least one additive selected fromthe group consisting of fillers, dyes, pigments, mold release agents,processing aids, carbon fiber, compounds that improve electricalconductivity, and compounds that improve thermal conductivity.

In one embodiment the graphite is expanded graphite.

Another aspect of the invention is directed to an automotive, aircraft,watercraft or aerospace part formed from the graphene-reinforced polymermatrix composites disclosed above. In one embodiment the part is anengine part.

Yet another aspect of the invention is directed to a graphene-reinforcedpolymer matrix composite as disclosed above, wherein the composite isprepared by a method comprising the steps of:

-   -   (a) distributing graphite microparticles into a molten        thermoplastic polymer phase comprising one or more of said        matrix polymers; and    -   (b) applying a succession of shear strain events to the molten        polymer phase so that the matrix polymers exfoliate the graphite        successively with each event until at least 50% of the graphite        is exfoliated to form a distribution in the molten polymer phase        of single- and multi-layer graphene nanoparticles less than 50        nanometers thick along a c-axis direction.

In one embodiment the graphite particles are prepared by crushing andgrinding a graphite-containing mineral to millimeter-sized dimensions,reducing the millimeter-sized particles to micron-sized dimensions, andextracting micron-sized graphite particles from the graphite-containingmineral. In one embodiment the graphite particles are distributed intothe molten polymer phase using a single screw extruder with axial flutedextensional mixing elements or spiral fluted extensional mixingelements. In one embodiment the graphite-containing molten polymer phaseis subjected to repeated extrusion to induce exfoliation of thegraphitic material and form the essentially uniform dispersion of thesingle- and multi-layer graphene nanoparticles in the thermoplasticpolymer matrix.

In certain embodiments, the thermoplastic polymer is selected frompolyetherether-ketones, polyether-ketones, polyphenylene sulfides,polyethylene sulfides, polyether-imides, polyvinylidene fluorides,polysulfones, polycarbonates, polyphenylene ethers or oxides,polyamides, aromatic thermoplastic polyesters, aromatic polysulfones,thermo-plastic polyimides, liquid crystal polymers, thermoplasticelastomers, polyethylenes (including high-density polyethylenes),polypropylenes, polystyrene, acrylics such as polymethylmethacrylate,polyacrylonitriles, polylactic acids (PLA), polyglycolic acid (PGA),polylactic-glycolic acid copolymers (PLGA), acrylonitrile butadienestyrene (ABS) copolymers, ultra-high-molecular-weight polyethylene,polytetrafluoroethylene, polyoxymethylene plastic, polyaryletherketones,polyvinylchloride, and mixtures of two or more thereof.

In specific embodiments the thermoplastic polymer is selected from thegroup consisting of polyamides, polystyrenes (PS), polyphenylenesulfides (PPS), high-density polyethylenes (HDPE). acrylonitrilebutadiene styrene (ABS) polymers, polyacrylonitriles, polylactic acids(PLA), polyglycolic acids (PGA) and polylactic-glycolic acid copolymers(PLGA). Polyamides include aliphatic polyamides, semi-aromaticpolyamides, and aromatic polyamides. Aliphatic polyamides contain noaromatic moieties. In one embodiment the aliphatic polyamides areselected from the group consisting of polyamide-6,6, polyamide-6(nylon-6), polyamide-6,9; polyamide-6,10; polyamide-6,12; polyamide-4,6;polyamide-11 (nylon-11), polyamide-12 (nylon-12) and other nylons. In aparticularly preferred embodiment the aliphatic polyamide ispolyamide-6,6, which is derived from hexamethylenediamine and adipicacid. Another useful aliphatic polyamide is PA-6, also known as nylon-6,which is polycaprolactam. Semi-aromatic polyamides contain a mixture ofaliphatic and aromatic moieties, and can be derived, for example, froman aliphatic diamine and an aromatic diacid. The semi-aromatic polyamidecan be a polyphthalamide such as PA-6T, which is derived fromhexamethylenediamine and tere-phthalic acid. Aromatic polyamides, alsoknown as aramids, contain aromatic moieties, and can be derived, forexample, from an aromatic diamine and an aromatic diacid. The aromaticpoly-amide can be a para-aramid such as those derived frompara-phenylenediamine and terephthalic acid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the morphology analysis of 2% graphite exfoliated inpolysulfone at mixing times of 3 minutes, 30 minutes, and 90 minutesaccording to an in situ exfoliation method of the present disclosure.

FIG. 2 illustrates micrographs of 90G-PMC at various scales andmagnification levels according to an in situ exfoliation method of thepresent disclosure.

FIG. 3 illustrates the morphology of SMG-PEEK_90 at (a) 10 μm scale and1,000 X, (b) 10 μm scale and 5,000 X, (c) 1 μm scale and 10,000 X, and(d) 1 μm scale and 50,000 X.

FIG. 4 illustrates the modulus of a graphite PA-6,6 composite aftervarious extrusion cycles.

FIG. 5 illustrates the peak stress of a graphite PA-6,6 composite aftervarious extrusion cycles.

FIG. 6 illustrates the impact energy of a graphite PA-6,6 compositeafter various extrusion cycles.

FIG. 7 illustrates the impact strength of an injection molded graphitePA-6,6 composite after various extrusion cycles.

FIG. 8 illustrates the adhesion on the step/edge of a graphite PA-6,6composite after cycle 5.

FIG. 9 illustrates the adhesion/bonding of PA-6,6 to graphene.

FIG. 10 displays the presence of cracks on the graphene surface of a 35%graphite in PA-6,6, cycle 3, composite, and a possible mode of surfacecrystallization by the polyamide. Crystalline graphene has a smoothsurface in such FESEM micrographs.

FIG. 11 displays transmission imaged at a relatively high acceleratingvoltage (10 kV at 5KX for (a) and 10 kV at 30KX (for (b)) of a 35%graphite in PA-6,6, cycle 1, composite.

FIG. 12 displays transmission imaged at 5 kV at 10KX for a 35% graphitein PA-6,6, cycle 3, composite.

FIG. 13 displays transmission imaged at 5 kV (a), 4 kV (b), 3 kV (c) and2 kV (d) at 5KX for a 35% graphite in PA-6,6, cycle 1, composite.

FIG. 14 displays SEM micrographs of G-PA66 Type I specimens, showinggood distribution of graphene flakes in the polymer matrix in (a)-(d),and a transparent graphene flake in (e).

FIG. 15 displays SEM micrographs of G-PA66 Type V specimens, showinggood distribution of graphene flakes in the polymer matrix in (a)-(d),and transparent graphene flakes in (e) and (f).

FIG. 16 shows graphs of tensile results for Type I PA66 and G-PA66specimens: (a) modulus, (b) stress and % strain at yield, and (c) stressand % strain at break.

FIG. 17 shows graphs of tensile results for Type V PA66 and G-PA66specimens: (a) modulus, (b) stress and % strain at yield, and (c) stressand % strain at break.

FIG. 18 displays notched Izod impact resistance of PA66, and G-PA66prepared using a continuous process.

FIG. 19 displays SEM micrographs displaying the morphology of G-PEEK atdifferent scales.

FIG. 20 shows graphs of tensile results for Type I specimens of PEEK andG-PEEK: (a) modulus, (b) stress and % strain at yield, and (c) stressand % strain at break.

FIG. 21 displays notched Izod impact resistance for PEEK and G-PEEK.

FIG. 22 shows a graph of the flexural modulus for G-PS specimens ascompared with polystyrene (PS).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This disclosure is not limited to the particular systems, methodologiesor protocols described, as these may vary. The terminology used in thisdescription is for the purpose of describing the particular versions orembodiments only, and is not intended to limit the scope.

As used in this document, the singular forms “a,” “an,” and “the”include plural reference unless the context clearly dictates otherwise.Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art. All publications mentioned in this document are incorporatedby reference. All sizes recited in this document are by way of exampleonly, and the invention is not limited to structures having the specificsizes or dimensions recited below. Nothing in this document is to beconstrued as an admission that the embodiments described in thisdocument are not entitled to antedate such disclosure by virtue of priorinvention. As used herein, the term “comprising” means “including, butnot limited to.”

The following term(s) shall have, for purposes of this application, therespective meanings set forth below:

The term “graphene” refers to the name given to a single layer of carbonatoms densely packed into a fused benzene-ring structure. Graphene, whenused alone, may refer to multi-layer graphene, graphene flakes, grapheneplatelets, and few-layer graphene or single-layer graphene in a pure anduncontaminated form.

The present invention provides a high efficiency mixing method totransform a polymer composite that contains well-crystallized graphiteparticles into nano-dispersed single- or multi-layer graphene particles.The method involves in situ exfoliation of the graphite layers bycompounding in a batch mixer or extruder that imparts repetitive, highshear strain rates. In both processes, longer mixing times provideenhanced exfoliation of the graphite into graphene nanoparticles withinthe polymer matrix composite (PMC). In addition, additives may be usedto promote sufficient graphene/polymer bonding, thereby yielding a lowdensity graphene-reinforced polymer matrix composite (G-PMC). The methodis low cost to produce a G-PMC that offers numerous property advantages,including improved mechanical properties such as increased specificstiffness and strength, enhanced electrical/thermal conductivity, andretention of optical transparency. Furthermore, these properties aretunable by modification of the process, vide infra.

The graphene-reinforced polymers may be used as electrodes forlightweight batteries. Other uses include composite boat hulls,aircraft, aerospace systems, transportation vehicles, light-weight armor(vehicular or personnel armor), pressure vessels, reactor chambers,spray coatings, polymer powders for 3-D printing, transparent electrodesfor electronic device touch screens, and the like. Addition of 1-2 wt %graphene to a polymer matrix imparts electrical conductivity, whilemaintaining optical transparency, thus enabling applications in solarpanels, flat-panel displays, and for static-discharge control inhospitals.

Repeated compounding during a batch mixing process or single screwextrusion is used to progressively transform the initialgraphite-particle dispersion into a uniform nano-dispersion of discretegraphene nanoparticles. In some cases, an inert gas or vacuum may beused during processing. The method is described herein as “mechanical”exfoliation to distinguish it from “chemical” exfoliation, which is theprimary thrust of much of the current research. An advantage of themechanical method is that contamination-free graphene-polymer interfacesare formed during high-shear mixing, thus ensuring good interfaceadhesion or bonding. Other advantages of in situ exfoliation are that itavoids making and handling graphene flakes, as well as avoiding the needto disperse them uniformly in the polymer matrix phase. Superior mixingproduces finer composite structures and very good particle distribution.

Depending on the number and duration of in situ shear strain events, themethod provides multi-layer graphene, graphene flakes, grapheneplatelets, few-layer graphene or single-layer graphene in a pure anduncontaminated form. Platelets have diamond-like stiffness and are usedfor polymer reinforcement. Graphene in any form increases polymertoughness by inhibiting crack propagation as reinforcement for polymers.Graphene may also be used as an additive to polymers and othercompositions to provide electrical and thermal conductivity. The thermalconductivity of graphene makes it a desirable additive for thermalmanagement for electronic devices and lasers.

Graphite, the starting material from which graphene is formed, iscomposed of a layered planar structure in which the carbon atoms in eachlayer are arranged in a hexagonal lattice. The planar layers are definedas having an “a” and a “b” axis, with a “c” axis normal to the planedefined by the “a” and “b” axes. The graphene particles produced by theinventive method have an aspect ratio defined by the “a” or “b” axisdistance divided by the “c” axis distance. Aspect ratio values for theinventive nanoparticles exceed 25:1 and typically range between 50:1 and1000:1.

The graphene may be produced as a graphene-polymer mixture suitable foruse as-is as a G-PMC that can be pelletized by conventional means forsubsequent fabrication processing. Alternatively higher concentrationsof graphite may be used at the outset to provide a graphene-polymermasterbatch in concentrated form that can also be pelletized and thenused to add graphene to polymer compositions as a reinforcing agent. Asa further alternative, the graphene may be separated from the polymer,for example, by combustion or selective dissolution, to provideessentially pure particles of graphene.

It should be understood that essentially any polymer inert to graphiteand capable of imparting sufficient shear strain to exfoliate graphenefrom the graphite may be used in the method of the present invention.Examples of such polymers include, but are not limited to,polyetherether-ketones (PEEK), polyetherketones (PEK), polyphenylenesulfides (PPS), polyethylene sulfide (PES), polyetherimides (PEI),polyvinylidene fluoride (PVDF), polysulfones (PSU), polycarbon-ates(PC), polyphenylene ethers, aromatic thermoplastic polyesters, aromaticpolysulfones, thermosplastic polyimides, liquid crystal polymers,thermoplastic elastomers, polyethylene, high-density polyethylene(HDPE), polypropylene, polystyrene (PS), acrylics such aspolymethyl-methacrylate (PMMA), polyacrylonitriles (PAN), acrylonitrilebutadiene styrene (ABS) co-polymers, and the like,ultra-high-molecular-weight polyethylene (UHMWPE),polytetrafluoro-ethylene (PTFE/TEFLON®), polyamides (PA), polylacticacids (PLA), polyglycolic acid (PGA), polylactic-glycolic acidcopolymers (PLGA), polyphenylene oxide (PPO), polyoxymethylene plastic(POM/Acetal), polyarylether-ketones, polyvinylchloride (PVC), mixturesthereof, and the like. Polymers capable of wetting the graphite surfacemay be used as well as high melting point, amorphous polymers inaccordance with the method of the present invention. The polylacticacids (PLA) can be chiral or racemic.

In specific embodiments the thermoplastic polymer is selected from thegroup consisting of polyamides, polystyrenes, a polyphenylene sulfides,high-density polyethylenes, acrylonitrile butadiene styrene (ABS)polymers, polyacrylonitriles, polylactic acids (PLA), polyglycolic acid(PGA) and polylactic-glycolic acid copolymers (PLGA). Polyamides includealiphatic polyamides, semi-aromatic polyamides, and aromatic polyamides.Aliphatic polyamides contain no aromatic moieties. In one embodiment thealiphatic polyamides are selected from the group consisting ofpolyamide-6,6 (nylon-6,6), polyamide-6 (nylon-6), polyamide-6,9;polyamide-6,10; polyamide-6,12; polyamide-4,6; polyamide-11 (nylon-11),polyamide-12 (nylon-12) and other nylons. Nylons are a well-known classof aliphatic polyamide derived from aliphatic diamines and aliphaticdiacids. Alternatively, other polyamides also classed as nylons arederived from ring-opening polymerization of a lactam, such as nylon-6(PA-6, polycaprolactam), derived from caprolactam. In a particularlypreferred embodiment the aliphatic polyamide is polyamide-6,6, which isderived from hexamethylenediamine and adipic acid. Semi-aromaticpolyamides contain a mixture of aliphatic and aromatic moieties, and canbe derived, for example, from an aliphatic diamine and an aromaticdiacid. The semi-aromatic polyamide can be a polyphthal-amide such asPA-6T, which is derived from hexamethylenediamine and terephthalic acid.Aromatic polyamides, also known as aramids, contain aromatic moieties,and can be derived, for example, from an aromatic diamine and anaromatic diacid. The aromatic polyamide can be a para-aramid such asthose derived from para-phenylenediamine and terephthalic acid. Arepresentative of the latter includes KEVLAR®.

Graphene-reinforced polymers according to the present inventiontypically contain between about 10 wt % and about 50 wt %, preferablyabout 20 wt % to about 40 wt %, more preferably about 25 wt % to about35 wt %, and most preferably about 30 to about 35 wt % graphene. Moretypically, the polymers contain between about 25 wt % and about 45 wt %graphene. One preferred embodiment contains 35 wt % graphene. Polymermasterbatches typically contain between about 30 and about 60 wt %graphene, and more typically between about 20 and about 50 wt %graphene.

The availability of graphite-rich mineral deposits, containingrelatively high concentrations (e.g., about 20%) of well-crystallizedgraphite, makes for a low cost and virtually inexhaustible source of rawmaterial. As discussed below, the extraction of graphite particles frommined material can be accomplished in a cost-effective manner. Syntheticgraphite of high purity and exception-al crystallinity (e.g., pyrolyticgraphite) may also be used for the same purpose. However, in this case,a batch mixing or extrusion compounding-induced exfoliation processcreates a laminated composite, in which the graphene nanoparticles areoriented over a relatively large area. Such laminated composites may bepreferred for specific applications.

For purposes of the present invention, graphite micro-particles aredefined as graphite in which at least 50% of the graphite consists ofmultilayer graphite crystals ranging between 1.0 and 1000 microns thickalong the c-axis of the lattice structure. Typically 75% of the graphiteconsists of crystals ranging between 100 and 750 microns thick. Expandedgraphite may also be used. Expanded graphite is made by forcing thecrystal lattice planes apart in natural flake graphite, thus expandingthe graphite, for example, by immersing flake graphite in an acid bathof chromic acid, then concentrated sulfuric acid. Expanded graphitesuitable for use in the present invention includes expanded graphitewith opened edges at the bilayer level, such as MESOGRAF.

Mechanical exfoliation of graphite within a polymer matrix may beaccomplished by a polymer processing technique that imparts repetitivehigh shear strain events to mechanically exfoliate graphitemicroparticles into multi- or single-layer graphene nanoparticles withinthe polymer matrix.

A succession of shear strain events is defined as subjecting the moltenpolymer to an alternating series of higher and lower shear strain ratesover essentially the same time intervals so that a pulsating series ofhigher and lower shear forces associated with the shear strain rate areapplied to the graphite particles in the molten polymer. Higher andlower shear strain rates are defined as a first higher, shear strainrate that is at least twice the magnitude of a second lower shear strainrate. The first shear strain rate will range between 100 and 10,000sec⁻¹. At least 1,000 to over 10,000,000 alternating pulses of higherand lower shear strain pulses are applied to the molten polymer to formthe exfoliated graphene nanoparticles. The number of alternating pulsesrequired to exfoliate graphite particles into graphene particles may bedependent on the original graphite particle dimensions at the beginningof this process, i.e., smaller original graphite particles may need alower number of alternating pulses to achieve graphene than largeroriginal graphite particles. This can be readily determined by one ofordinary skill in the art guided by the present specification withoutundue experimentation.

After high-shear mixing, the graphene flakes are uniformly dispersed inthe molten polymer, are randomly oriented, and have high aspect ratio.Orientation of the graphene may be achieved by many different methods.Conventional drawing, rolling, and extrusion methods may be used todirectionally align the graphene within the PMC fiber, filament, ribbon,sheet, or any other long-aspect shape. The method to fabricate andcharacterize a G-PMC is comprised of four main steps including:

-   -   1. Extraction of crystalline graphite particles from a mineral        source;    -   2. Incorporation of the extracted graphite particles into a        polymer matrix phase and conversion of the graphite-containing        polymer into a graphene-reinforced polymer matrix composite        (G-PMC) by a high efficiency mixing/exfoliation process;    -   3. Morphology analysis to determine the extent of mechanical        exfoliation and distribution of multi-layer graphene and        graphene nanoparticles; and    -   4. X-ray diffraction analysis to determine multi-layer graphene        or graphene crystal size as a function of mechanical        exfoliation.

Highly crystalline graphite may be extracted from graphite ore by amulti-step process, as described below.

-   -   1. Crushing: A drilled rod of graphite ore from the mine may be        placed in a vice and crushed.    -   2. Grinding: The crushed graphite ore may be then ground by        mortar and pestle.    -   3. Size Reduction: The ground graphite ore may be placed in a        sieve with a 1-mm mesh size and size reduced. Larger pieces that        do not pass through the screen may be ground by mortar and        pestle and then size reduced through the 1-mm mesh size again.        Eventually, all of the material passed through the 1-mm mesh        size to obtain graphite ore powder.    -   4. Density Separation by Water: The 1-mm sized powder may be        placed in a column filled with water and agitated until a clear        separation formed between the more dense portions of the solids        and the less dense portions. Graphite is near the density of        water (1 g/cm³), while silicon is much more dense (2.33 g/cm³).        The uppermost materials are siphoned off with the water and then        dried. The dried powder graphite is referred to as Separated        Mineral Graphite (SMG).

In commercial practice, very large crushing and grinding machines areavailable to produce tonnage quantities of mixed powders, from which thegraphite component can be separated by standard floatation methods.

Thus, one aspect of the invention is directed to an in situ exfoliationmethod of fabricating a G-PMC. In this method, a polymer that isuniformly blended with micron-sized crystalline graphite particles issubjected to repeated compounding-element processing during batch mixingor extrusion at a temperature where the polymer adheres to the graphiteparticles. Typical polymers have a heat viscosity (without graphite)greater than 100 cps at the compounding temperature. The compoundingtemperature will vary with the polymer and can range between roomtemperature (for polymers that are molten at room temperature) and 600°C. Typical compounding temperatures will range between 180° C. and 400°C.

In one embodiment, the extrusion compounding elements are as describedin U.S. Pat. No. 6,962,431, the disclosure of which is incorporatedherein by reference, with compounding sections, known as axial flutedextensional mixing elements or spiral fluted extensional mixingelements. The compounding sections act to elongate the flow of thepolymer and graphite, followed by repeated folding and stretching of thematerial. This results in superior distributive mixing, which in turn,causes progressive exfoliation of the graphite particles into discretegraphene nanoparticles. Batch mixers may also be equipped withequivalent mixing elements. In another embodiment, a standard-typeinjection molding machine is modified to replace the standard screw witha compounding screw for the purpose of compounding materials as thecomposition is injection molded. Such a device is disclosed in US2013/0072627, the entire disclosure of which is incorporated herein byreference.

Thus, the effect of each compounding pass is to shear-off graphenelayers one after the other, such that the original graphite particlesare gradually transformed into a very large number of graphenenanoparticles. After an appropriate number of such passes, the finalresult is a uniform dispersion of discrete graphene nanoparticles in thepolymer matrix phase. Longer mixing times or a higher number of passesthrough the compounding elements provides smaller graphite crystal sizeand enhanced exfoliation of graphite into graphene nanoparticles withinthe polymer matrix; however, the shear events should not be of aduration that would degrade the polymer.

As the content of graphene nanoparticles increases during multi-passextrusion, the viscosity of the polymer matrix increases due to theinfluence of the growing number of polymer/graphene interfaces. Toensure continued refinement of the composite structure, the extrusionparameters are adjusted to compensate for the higher viscosity of thecomposite.

Automated extrusion systems are available to subject the compositematerial to as many passes as desired, with mixing elements as describedin U.S. Pat. No. 6,962,431, and equipped with a re-circulating stream todirect the flow back to the extruder input. Since processing of thegraphene-reinforced PMC is direct and involves no handling of grapheneparticles, fabrication costs are low.

In order to mechanically exfoliate graphite into multi-layer grapheneand/or single-layer graphene, the shear strain rate generated in thepolymer during processing must cause a shear stress in the graphiteparticles greater than the critical stress required to separate twolayers of graphite, or the interlayer shear strength (ISS). The shearstrain rate within the polymer is controlled by the type of polymer andthe processing parameters, including the geometry of the mixer,processing temperature, and speed in revolutions per minute (RPM).

The required processing temperature and speed (RPM) for a particularpolymer is determinable from polymer rheology data given that, at aconstant temperature, the shear strain rate ({dot over (γ)}) is linearlydependent upon RPM, as shown by Equation 1. The geometry of the mixerappears as the rotor radius, r, and the space between the rotor and thebarrel, Δr.

$\begin{matrix}{\overset{.}{\gamma} = {\left( \frac{2\; \pi \; r}{\Delta \; r} \right)\left( \frac{RPM}{60} \right)}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Polymer rheology data collected for a particular polymer at threedifferent temperatures provides a log shear stress versus log shearstrain rate graph. The ISS of graphite ranges between 0.2 MPa and 7 GPa,but a new method has quantified the ISS at 0.14 GPa. Thus, tomechanically exfoliate graphite in a polymer matrix during processing,the required processing temperature, shear strain rate, and RPM isdeterminable for a particular polymer from a graph of the log shearstress versus the log shear strain rate, collected for a polymer at aconstant temperature, so that the shear stress within the polymer isequal to or greater than the ISS of graphite. Under typical processingconditions, polymers have sufficient surface energy to behave like thesticky side of adhesive tape, and thus are able to share the shearstress between the polymer melt and the graphite particles.

Thus, one aspect of the invention is directed to a graphene-reinforcedpolymer matrix composite comprising an essentially uniform distributionin a thermoplastic polymer matrix of between about 10 wt % and about 50wt %, preferably about 20 wt % to about 40 wt %, more preferably about25 wt % to about 35 wt %, and most preferably about 30 wt % to about 35wt % of total composite weight of particles selected from the groupconsisting of graphite microparticles, single-layer graphenenanoparticles, multi-layer graphene nanoparticles, and combinations oftwo or more thereof where at least 50 wt % of the particles consist ofsingle- and/or multi-layer graphene nanoparticles less than 50nanometers thick along a c-axis direction; and the thermo-plasticpolymer is selected from polyamides, polystyrenes, polyphenylenesulfides high-density polyethylene, ABS polymers, polyacrylonitriles,polylactic acids, polyglycolic acids, polylactic-glycolic acidcopolymers (PLGA), and mixtures of two or more thereof.

According to one embodiment the graphene-reinforced polymer matrixcomposite contains an essentially uniform distribution between about 1and about 45% of total composite weight of graphite and grapheneparticles. In another embodiment the graphene-reinforced polymer matrixcomposite contains between about 3 and about 40% of total compositeweight of graphite and graphene particles. In another embodiment thegraphene-reinforced polymer matrix composite contains between about 5and about 35% of total composite weight of graphite and grapheneparticles. In another embodiment the graphene-reinforced polymer matrixcomposite contains between about 7 and about 30% of total compositeweight of graphite and graphene particles.

As defined herein, “essentially uniform” denotes that the grapheneparticles well-distributed throughout the molten thermoplastic polymerphase, so that individual aliquots of the composite contain the sameamount of graphene within about 10 wt % of the average value, preferablywithin about 5 wt % of the average value, more preferably within about 1wt % of the average value. The thermoplastic polymers are of a type andgrade sufficient to exfoliate graphene from graphite under shear strain.In one embodiment the thermoplastic polymer comprises a polyamide. In apreferred embodiment the polyamide is selected from aliphaticpolyamides, semi-aromatic poly-amides, aromatic polyamides andcombinations of two or more thereof. In a one preferred embodiment thepolyamide is an aliphatic polyamide selected from the group consistingof polyamide-6,6; polyamide-6,9; polyamide-6,10; polyamide-6,12;polyamide-4,6; polyamide-6 (nylon-6); polyamide-11 (nylon-11),polyamide-12 (nylon-12) and other nylons. In a particularly preferredembodiment the aliphatic polyamide is polyamide-6,6, which is also knownas PA-6,6 or nylon-6,6, derived from hexamethylenediamine and adipicacid. Another useful polyamide is PA-6, also known as nylon-6, which ispolycaprolactam.

In another embodiment of the above graphene-reinforced polymer matrixcomposite, the polyamide is a semi-aromatic polyamide. In a preferredembodiment the semi-aromatic polyamide is a polyphthalamide such asPA-6T, derived from hexamethylenediamine and terephthalic acid.

In another embodiment of the above graphene-reinforced polymer matrixcomposite, the poly-amide is an aromatic polyamide, also known as anaramid. In a preferred embodiment the aromatic polyamide is apara-aramid such as KEVLAR®, derived from para-phenylenediamine andterephthalic acid.

In one embodiment of the graphene-reinforced polymer matrix compositesas disclosed above, the graphite may be doped with other elements tomodify a surface chemistry of the exfoliated graphene nanoparticles.Preferably the graphite is expanded graphite. Specifically andprefer-ably, a surface chemistry or nanostructure of the dispersedgraphite may be modified to bond with the polymer matrix to increasestrength and stiffness of the graphene-reinforced composite. In oneembodiment, directional alignment of the graphene nanoparticles is usedto obtain one-, two- or three-dimensional reinforcement of the polymermatrix phase. In one embodiment the polymer chains are inter-molecularlycross-linked by single- or multi-layer graphene sheets having carbonatoms with reactive bonding sites on the edges of said sheets.

In one aspect of the invention, the above graphene-reinforced polymermatrix composite further comprises at least one additive selected fromfillers, dyes, pigments, mold release agents, processing aids, carbonfiber, compounds that improve electrical conductivity, and compoundsthat improve thermal conductivity.

Another aspect of the invention is directed to an automotive, aircraft,watercraft or aerospace part formed from the graphene-reinforced polymermatrix composites disclosed above. In one embodiment the part is anengine part.

Yet another aspect of the invention is directed to a graphene-reinforcedpolymer matrix composite as disclosed above, wherein the composite isprepared by a method comprising the steps of:

-   -   (a) distributing graphite microparticles into a molten        thermoplastic polymer phase comprising one or more of said        matrix polymers; and    -   (b) applying a succession of shear strain events to the molten        polymer phase so that the matrix polymers exfoliate the graphite        successively with each event until at least 50% of the graphite        is exfoliated to form a distribution in the molten polymer phase        of single- and multi-layer graphene nanoparticles less than 50        nanometers thick along a c-axis direction.

In one embodiment the graphite particles are prepared by crushing andgrinding a graphite-containing mineral to millimeter-sized dimensions,reducing the millimeter-sized particles to micron-sized dimensions, andextracting micron-sized graphite particles from the graphite-containingmineral. In one embodiment the graphite particles are distributed intothe molten polymer phase using a single screw extruder with axial flutedextensional mixing elements or spiral fluted extensional mixingelements. In one embodiment the graphite-containing molten polymer phaseis subjected to repeated extrusion to induce exfoliation of thegraphitic material and form the essentially uniform dispersion of thesingle- and multi-layer graphene nanoparticles in the thermoplasticpolymer matrix.

In another embodiment, a cross-linked G-PMC is formed by a methodincluding distributing graphite microparticles into a moltenthermoplastic polymer phase comprising one or more molten thermoplasticpolymers. A succession of shear strain events, as illustrated in theexamples, is then applied to the molten polymer phase so that the moltenpolymer phase exfoliates the graphene successively with each event untila lower level of graphene layer thickness is achieved, after which pointripping and tearing of exfoliated multilayer graphene sheets occurs andproduces reactive edges on the multilayer sheets that react with andcross-link the thermoplastic polymer.

Thus, activated graphene is formed as the graphene fractures acrossbasal plane and offers potential sites for cross-linking to the matrixor attaching other chemically unstable groups for functionalization.Therefore, the cross-linking is performed under exclusion of oxygen,preferably under an inert atmosphere or a vacuum, so that the reactiveedges do not oxidize or other-wise become unreactive. Forming covalentbonds between graphene and the matrix significantly increases thecomposite strength. Polymers that cross-link when subjected to themethod of the present invention include polymers subject to degradationby ultraviolet (UV) light. This includes polymers containing aromatic,e.g., benzene rings, such as polystyrene, polymers containing tertiarycarbons, such as polypropylene and the like, polymers containingbackbone oxygens, such as poly(alkylene oxides), and the like.

In another embodiment, the cross-linked G-PMC can be ground intoparticles and blended with non-cross-linked host polymers to serve astoughening agents for the host polymer. The non-cross-linked polymeracquires the properties of the cross-linked polymer because of chainentanglement between the two polymer species. The present inventiontherefore also includes cross-linked polymers of the present inventionin particulate form that can be blended with other polymers to form ahigh strength composite. In one embodiment cross-linked polystyrene andpolymethyl methacrylate (PMMA) particles of the present invention can beused as toughening agents for host polymers. Compositions according tothe present invention include host thermos-plastic polymers toughenedwith between about 1 and about 75% by weight of the cross-linked polymerparticles of the present invention. In one embodiment, the host polymersare toughened with between about 10 and about 50% by weight of thecross-linked polymer particles.

In certain embodiments, the thermoplastic host polymer is an aromaticpolymer. As defined herein the term “aromatic polymer” refers to apolymer comprising aromatic moieties, either as part of the polymerbackbone or as substituents attached to the polymer backbone, optionallyvia a linker. Linkers include linear or branched alkylene groups, suchas methylene, ethylene, and propylene, linear or branched heteroalkylenegroups, such as —OCH₂—, —CH₂O—, —OCH₂CH₂—, —CH₂CH₂O—, —OCH₂CH₂CH₂—,—CH₂OCH₂—, —OCH(CH₃)—, —SCH₂—, —CH₂S—, —NRCH₂—, —CH₂NR—, and the like,where the heteroatom is selected from the groups consisting of oxygen,nitrogen and sulfur, and R is selected from hydrogen and lower alkyl.Linkers can also be heteroatomic, such as —O—, —NR— and —S—. When thelinkers contain sulfur, the sulfur atom is optionally oxidized. Thearomatic moieties are selected from monocyclic, e.g. phenyl, andpolycyclic moieties, e.g. indole naphthyl, anthracene, etc., and areoptionally substituted with amino, NHR, NR₂, halogen, nitro, cyano,alkylthio, alkoxy, alkyl, haloalkyl, CO₂R where R is defined as above,and combinations of two or more thereof. The aromatic moieties can alsobe heteroaryl, comprising one to three heteroatoms selected from thegroup consisting of oxygen, nitrogen and sulfur, and optionallysubstituted as described above. The aromatic polymer preferablycomprises phenyl groups, optionally substituted as disclosed above,either as part of the polymer backbone or as substituents on thebackbone, the latter optionally through a linker, as disclosed above. Incertain embodiments the optionally substituted phenyl groups arecontained within the polymer backbone as optionally substitutedphenylene groups. In certain other embodiments the optionallysubstituted phenyl groups are substituents on the polymer backbone,optionally connected through a linker, as described above.

Examples of thermoplastic host polymers include, but are not limited to,polyetheretherketone (PEEK), polyetherketone (PEK), polyphenylenesulfide (PPS), polyethylene sulfide (PES), poly-etherimide (PEI),polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC),poly-phenylene ether, aromatic thermoplastic polyesters, aromaticpolysulfones, thermoplastic poly-imides, liquid crystal polymers,thermoplastic elastomers, polyethylene, high-density poly-ethylene(HDPE), polypropylene, polystyrene (PS), acrylics such aspolymethylmethacrylate (PMMA), polyacrylonitriles (PAN), acrylonitrilebutadiene styrene (ABS) copolymers, and the like,ultra-high-molecular-weight polyethylene (UHMWPE),polytetrafluoroethylene (PTFE/TEFLON®), polyamides (PA), polylacticacids (PLA), polyglycolic acid (PGA), polylactic-glycolic acidcopolymers (PLGA), poly-phenylene oxide (PPO), polyoxymethylene plastic(POM/Acetal), polyimides, polyarylether-ketones, polyvinylchloride(PVC), acrylics, mixtures thereof, and the like. When the thermoplastichost polymer and the cross-linked polymer are the same polymer species,the cross-linked polymer particles are essentially a concentratedmasterbatch of the degree of cross-linked species desired to beintroduced to the polymer formulation.

In specific embodiments the thermoplastic host polymer is selected fromthe group consisting of polyamides, polystyrenes, polyphenylenesulfides, high-density polyethylenes, acrylonitrile butadiene styrene(ABS) polymers, polyacrylonitriles, polylactic acids (PLA), polyglycolicacid (PGA) and polylactic-glycolic acid copolymers (PLGA). Polyamidesinclude aliphatic polyamides, semi-aromatic polyamides, and aromaticpolyamides. Aliphatic polyamides contain no aromatic moieties. In oneembodiment the aliphatic polyamides are selected from the groupconsisting of polyamide-6,6 (nylon-6,6), polyamide-6 (nylon-6),polyamide-6,9; polyamide-6,10; polyamide-6,12; polyamide-4,6;polyamide-11 (nylon-11), polyamide-12 (nylon-12) and other nylons.Nylons are a well-known class of aliphatic poly-amide derived fromaliphatic diamines and aliphatic diacids. Alternatively, otherpolyamides also classed as nylons are derived from ring-openingpolymerization of a lactam, such as nylon-6 (PA-6, polycaprolactam),derived from caprolactam. In a particularly preferred embodiment thealiphatic polyamide is polyamide-6,6, which is derived fromhexamethylenediamine and adipic acid. Semi-aromatic polyamides contain amixture of aliphatic and aromatic moieties, and can be derived, forexample, from an aliphatic diamine and an aromatic diacid. Thesemi-aromatic polyamide can be a polyphthal-amide such as PA-6T, whichis derived from hexamethylenediamine and terephthalic acid. Aromaticpolyamides, also known as aramids, contain aromatic moieties, and can bederived, for example, from an aromatic diamine and an aromatic diacid.The aromatic polyamide can be a para-aramid such as those derived frompara-phenylenediamine and terephthalic acid. A representative of thelatter includes KEVLAR®.

In one embodiment, a G-PMC is formed by distributing graphitemicroparticles into a molten thermoplastic polymer phase and applying asuccession of shear strain events to the molten polymer phase so thatthe molten polymer phase exfoliates the graphite successively with eachevent until at least 50% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 50 nanometers thick along a c-axisdirection. In other embodiments, the succession of shear strain eventsmay be applied until at least 90% of the graphite is exfoliated to forma distribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection. In other embodiments, the succession of shear strain eventsmay be applied until at least 80% of the graphite is exfoliated to forma distribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection. In other embodiments, the succession of shear strain eventsmay be applied until at least 75% of the graphite is exfoliated to forma distribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection. In other embodiments, the succession of shear strain eventsmay be applied until at least 70% of the graphite is exfoliated to forma distribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection. In other embodiments, the succession of shear strain eventsmay be applied until at least 60% of the graphite is exfoliated to forma distribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the graphene-reinforced polymer matrix compositeconsists of graphite cross-linked with polymers selected from the groupconsisting of polyetheretherketones (PEEK), polyetherketones (PEK),polyphenylene sulfides (PPS), polyethylene sulfides (PES),polyether-imides (PEI), polyvinylidene fluoride (PVDF), polycarbonates(PC), polyphenylene ethers, aromatic thermoplastic polyesters,thermoplastic polyimides, liquid crystal polymers, thermo-plasticelastomers, polyethylene, high-density polyethylene (HDPE),polypropylene, polystyrene (PS), acrylics such as polymethylmethacrylate(PMMA), polyacrylonitriles (PAN), acrylonitrile butadiene styrene (ABS)copolymers, and the like, ultra-high-molecular-weight polyethylene(UHMWPE), polytetrafluoroethylene (PTFE/TEFLON®), polyamides (PA),polylactic acids (PLA), polyglycolic acid (PGA), polylactic-glycolicacid copolymers (PLGA), polyphenylene oxide (PPO), polyoxymethyleneplastic (POM/Acetal), polyaryletherketones, polyvinylchloride (PVC),mixtures thereof, and the like.

In specific embodiments the thermoplastic polymer is selected from thegroup consisting of polyamides, acrylonitrile butadiene styrene ABSpolymers, polyacrylonitriles, polystyrenes (PS), polyphenylene sulfides(PPS), high-density polyethylenes (HDPE), polylactic acids (PLA),polyglycolic acid (PGA) and polylactic-glycolic acid copolymers (PLGA).Polyamides include aliphatic polyamides, semi-aromatic polyamides, andaromatic polyamides. Aliphatic poly-amides contain no aromatic moieties.In one embodiment the aliphatic polyamides are selected from the groupconsisting of polyamide-6,6 (nylon-6,6), polyamide-6 (nylon-6),polyamide-6,9; polyamide-6,10; polyamide-6,12; polyamide-4,6;polyamide-11 (nylon-11), polyamide-12 (nylon-12) and other nylons.Nylons are a well-known class of aliphatic polyamide derived fromaliphatic diamines and aliphatic diacids. Alternatively, otherpolyamides also classed as nylons are derived from ring-openingpolymerization of a lactam, such as nylon-6 (PA-6, poly-caprolactam),derived from caprolactam. In a particularly preferred embodiment thealiphatic polyamide is polyamide-6,6, which is derived fromhexamethylenediamine and adipic acid. Semi-aromatic polyamides contain amixture of aliphatic and aromatic moieties, and can be derived, forexample, from an aliphatic diamine and an aromatic diacid. Thesemi-aromatic polyamide can be a polyphthal-amide such as PA-6T, whichis derived from hexamethylenedi-amine and terephthalic acid. Aromaticpolyamides, also known as aramids, contain aromatic moieties, and can bederived, for example, from an aromatic diamine and an aromatic diacid.The aromatic polyamide can be a para-aramid such as those derived frompara-phenylenediamine and terephthalic acid. A representative of thelatter includes KEVLAR®.

In other embodiments, the graphene-reinforced polymer matrix compositecomprises graphite cross-linked with a polyamide. Preferably thepolyamide is an aliphatic or a semi-aromatic polyamide. More preferablythe polyamide is an aliphatic polyamide selected from the groupconsisting of polyamide-6,6; polyamide-6 (nylon-6); polyamide-6,9;polyamide-6,10; polyamide-6,12; polyamide-4,6; polyamide-11 (nylon-11),polyamide-12 (nylon-12) and other nylons; particularly PA-6,6(nylon-6,6). Preferably the graphene-reinforced polymer matrix compositecontains about 35% graphite prior to in situ exfoliation of graphene. Apolyamide that is cross-linked in this manner will have very highspecific strength properties and is suitable for automotive, aviation,nautical and aerospace uses. The present invention therefore alsoincludes automotive, aircraft, watercraft and aerospace parts fabricatedfrom the cross-linked polyamide of the present invention, which canreplace heavier metal parts without loss of mechanical or hightemperature properties. For example, cross-linked polyamide can be usedin engine components such as pistons, valves, cam shafts, turbochargersand the like because of its high melting point and creep resistance.Forming the rotating portions of the turbine and compressor parts of aturbocharger, including the respective blades, from the cross-linkedpolyamide of the present invention will reduce turbocharger lag becauseof the resulting weight reduction. Other advantages are obtained byforming the rotating portions of the turbine and compressor of jetengines from a cross-linked polyamide of the present invention.

EXAMPLES

The present invention is further illustrated by the following examples,which should not be construed as limiting in any way.

Materials

Raw graphite was extracted from the ground, crushed to powder, and floatseparated to obtain Separated Mineral Graphite (“SMG”).

Example 1. Preparation of Graphene-Reinforced Polysulfone (G-PSU)

In one embodiment, a small scale extension mixer with a 10-gram capacitywas used to compound 2% of SMG with Udel P-1700 Polysulfone (PSU) at332° C. (630° F.) and under vacuum for 3, 30, and 90 minutes. The methodis described below. Samples collected for characterization after eachlength of time are referred to as 3G-PMC, 30G-PMC, 90G-PMC.

-   -   1. 9.8 grams of PSU were added to the mixer and allowed to        become molten.    -   2. 0.2 grams of SMG were added to the molten PSU and mixed.    -   3. After 3 minutes of mixing time, 3 grams of the G-PMC were        extruded out of the mixer and collected for characterization.    -   4. 3 grams of 2% SMG in PSU was added to the mixer and mixed.    -   5. After 30 minutes of mixing time, 3 grams of the G-PMC were        extruded out of the mixer and collected for characterization.    -   6. 3 grams of 2% SMG in PSU was added to the mixer and mixed.    -   7. After 90 minutes of mixing time, 3 grams of the G-PMC were        extruded out of the mixer and collected for characterization.

Example 2. Morphology Analysis

A Zeiss Sigma Field Emission Scanning Electron Microscope (FESEM) withOxford EDS was used to determine the degree of mechanical exfoliation ofgraphite into multi-layer graphene or graphene nanoparticles and thethickness of these particles. An accelerating voltage of 3 kV andworking distance of approximately 8.5 mm was used during viewing. Priorto viewing, specimens from each sample of 3G-PMC, 30G-PMC, and 90G-PMCwere notched, cryogenically fractured to produce a flat fracturesurface, placed under vacuum for at least 24 hours, gold coated, andstored under vacuum.

Morphology Results

The morphology of each sample, 3G-PMC, 30G-PMC, and 90G-PMC, at threedifferent scales (magnification) is shown in FIG. 1. In (a-c), a 20 μmscale and 1,000× magnification shows good distribution of multi-layergraphene or graphene within the PSU matrix at each mixing time. In(d-f), a 1 μm scale and 10,000× magnification and (g-i), a 1 μm scaleand 50,000× magnification shows mechanically exfoliated graphite withinthe PSU matrix. In (d-i), micro-folding of the multi-layer graphene orgraphene is evident, as well as good bonding between the graphenenanoparticles and the polymer matrix.

The 90G-PMC sample, the sample mixed for the longest time and exposed tothe most repetitive shearing, exhibits superior mechanical exfoliationand the smallest crystal size. As shown in FIG. 2, mechanicalexfoliation has reduced the graphene nanoparticle thickness in the90G-PMC sample to 8.29 nm.

Example 3. X-ray Diffraction Analysis (XRD)

XRD analysis on each sample of 3G-PMC, 30G-PMC, and 90G-PMC includesfour steps: (1) sample preparation, (2) diffraction pattern acquisition,(3) profile fitting, and (4) out-of-plane (D) crystallite sizescalculation according to the Debye-Scherrer equation.

-   -   1. The samples for XRD analysis were prepared by pressing thin        films of each sample 3G-PMC, 30G-PMC, and 90G-PMC at 230° C. and        5,500 psi over a 2 minute time period. Each sample was        positioned between aluminum sheets prior to pressing using a        Carver Uniaxial Press with heated platens.    -   2. Diffraction patterns of the pressed films were acquired using        a Philips XPert powder Diffractometer with sample changer        (Xpert) at 40 kV and 45 mA with an incident slit thickness of        0.3 mm from 4°-70° 2θ and a step size of 0.02° 2θ.    -   3. Diffraction patterns were uploaded into WinPLOTR Powder        diffraction graphics tool, without background editing or profile        adjustments prior to peak fitting. Single peak fitting was        applied at a 2θ range of 26°-27.5°, using a pseudo-Voigt        function and taking into account a global FWHM, global eta        (proportion of Lorentz), and linear background. Single peak        fitting of the profile provides the full width at half maximum        (FWHM) of the relevant peak.

The average out-of-plane crystallite size (D) (sometimes referred to asalong the c-axis, and proportional to the number of graphene layerswhich are stacked) is calculated using the Debye-Scherrer Equation andthe (002) FWHM values, for which λ, is the X-ray wavelength, coefficientK=0.89, β is the FWHM in radians, and θ is the diffraction angle. Thed-spacing is also calculated.

$\begin{matrix}{D = \frac{K\; \lambda}{\beta \; \cos \; \theta}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

X-ray Diffraction Results

The Debye-Scherrer equation was applied to the FWHM and d-spacingresults obtained from the X-ray diffraction patterns for 3G-PMC,30G-PMC, and 90G-PMC to provide the crystal thickness (D) of themulti-layer graphene or graphene nanoparticles. The XRD results andcrystal thickness appear in Table 1. For the 3G-PMC, 30G-PMC, and90G-PMC samples, the crystal thickness is 40 nm, 31 nm, and 23 nm; theFWHM is 0.202°, 0.257°, and 0.353°; and the d-spacing is 3.361 nm, 3.353nm, and 3.387 nm, respectively. The FWHM increases with mixing time, andcrystal thickness decreases with mixing time, which indicates thatmechanical exfoliation of the graphite to multi-layer graphene orgraphene is occurring and is enhanced over longer mixing times. Thedecrease in crystal size is a function of FWHM.

TABLE 1 Debye-Scherrer Equation applied to the average XRD results fromeach 2% Graphite Exfoliated in PSU sample mixed for 3 min, 30 min, and90 min Average D - Crystal Thickness (nm) Mixing Time (d 002) FWHM Alongc-Axis Sample (min) (nm) (degrees) Direction 3G-PMC 3 0.3361 0.202 4030G-PMC 30 0.3353 0.257 31 90G-PMC 90 0.3387 0.353 23

Example 4. Graphene Modification

Mechanical exfoliation of the graphite into multi-layer graphene orgraphene as a result of the repetitive shear strain action in thepolymer processing equipment generates dangling primary and secondarybonds that provide the opportunity for various chemical reactions tooccur, which can be exploited to obtain property enhancement of theG-PMC. This represents an advance over prior art conventional methodsforming graphene oxides, where the dangling primary and secondary bondscovalently bond with oxygen, which typically remain in these positionseven after the graphene oxide is reduced.

For example, chemical reactions that covalently attach these danglingbonds from the multi-layer graphene or graphene nanoparticles to thepolymer matrix would provide superior mechanical properties of theG-PMC. Alternatively, electrical conductivity may be enhanced bychemically linking appropriate band gap materials at the graphenenano-particle edges or by coordinating with conductive metals such asgold, silver, copper, and the like. The graphene-reinforced polymer maythen be added to polymers or other compositions to provide or increaseelectrical conductivity. The bonds may also be coordinated to metals,such as platinum and palladium, to provide a catalyst, with thegraphene-reinforced polymer serving as a catalyst support. Other formsof functionalized graphene are disclosed in U.S. Pat. No. 8,096,353, thedisclosure of which is incorporated herein by reference.

The method of the present invention is particularly advantageous becausein situ functionalization reactions may be performed during theexfoliation process via one-pot reactive compounding.

Mechanical exfoliation successfully converted 2% graphite melt-blendedwith PSU into a G-PMC using a repetitive shearing action in the SmallScale Extension Mixer by Randcastle Extrusion Systems, Inc.(“Randcastle”). Results may be improved by machine modification toincrease shear; for example, by using a larger diameter mixing elementto increase rotational speed and/or by minimizing the spacing betweenthe mixing element and the cylinder wall.

Example 5. Modified Randcastle Extrusion System's Small Scale ExtensionMixer

The design of the existing small batch mixer may be modified to providehigher shear rate, which in turn provides superior mechanicalexfoliation of graphite within the polymer matrix. The shear rate, {dotover (γ)}, is calculated according to Equation 1, where r is the toolingradius and Δr is the clearance for compounding. Machine modificationsare listed in Table 2, along with the maximum achievable shear rate. Thenewly designed mixer has a maximum shear rate 22 times that of thecurrent mixer, which will provide enhanced mechanical exfoliation ofgraphite within a polymer matrix at shorter lengths of time. In otherwords, the crystal size, D, may be reduced to smaller dimensions in amore efficient length of time.

TABLE 2 Modifications of the Randcastle Extrusion System's Small ScaleExtension Mixer to provide enhanced mechanical exfoliation CurrentRandcastle Improved Randcastle Mixer Mixer Tooling Radius (inches) 0.5 1Clearance for Compounding, Δr (in) 0.04 0.01 Maximum RPM 100 360 MaximumShear Strain Rate (sec⁻¹) 133 2900

Modified Single Screw Extrusion:

Randcastle has made modifications to the extruder screw that will betterenable mechanical exfoliation of the graphite into multi-layer grapheneor graphene in a polymer matrix to fabricate a G-PMC.

Example 6A. Graphene-Reinforced PEEK (G-PEEK)

PEEK has a specific gravity of 1.3, a melt flow of 3 g/10 min (400° C.,2.16 kg), a glass transition temperature at 150° C., and a melting pointat 340° C. The tensile modulus and strength are 3.5 GPa and 95 MPa,respectively. Prior to the creation of the xG-PMC in this example, SMGand PEEK were dried for approximately 12 hours at 100° C. and 150° C.,respectively.

In this example, SMG was blended with PEEK using a Randcastlemicro-batch mixer with a 10-gram capacity at 360° C. (680° F.) and 100RPM under a nitrogen blanket, according to the following steps:

-   -   PEEK_3—To create a control sample, 10 grams of PEEK was added to        the mixer. After three minutes of mixing time, the port was        opened to allow PEEK to flow out as extrudate and 2.6 grams were        extruded out until no more material was able to flow.    -   SMG-PEEK_3—To create a weight composition ratio of 2-98%        SMG-PEEK, 2.4 g of PEEK and 0.2 g of SMG were added to the        mixer. After three minutes of mixing time, the port was opened        to allow G-PMC to flow out as extrudate and 1.96 g were extruded        out until no more material was able to flow.    -   SMG-PEEK_30—To maintain the 2-98 wt % composition ratio, 1.92 g        of PEEK and 0.04 g of SMG were added to the mixer. After 30        minutes of mixing time, the port was opened to allow G-PMC to        flow out as extrudate and 0.94 g were extruded out until no more        material was able to flow.    -   SMG-PEEK_90—To maintain the 2-98 wt % composition ratio, 0.92 g        of PEEK and 0.02 g of SMG were added to the mixer. After 90        minutes of mixing time, the port was opened to allow G-PMC to        flow out as extrudate, however, no more material was able to        flow.

The experiment was terminated and the mixer opened. Under visualobservation, the G-PMC did not appear as a standard molten polymer, butrather was in a rubber-like, fibrous form.

Example 6B. Graphene-Reinforced PEEK (G-PEEK)

In this example, SMG and PEEK were processed in a Randcastle micro-batchmixer with a 100-gram capacity at 360° C. (680° F.) and 30 RPM under anitrogen blanket, according to the following steps:

-   -   PEEK_90—To create a control sample, 100 g of PEEK was added to        the mixer. After 90 minutes of mixing time, the port was opened        to allow PEEK to flow out as extrudate and 28.5 g were extruded        out until no more material was able to flow.    -   SMG-PEEK_25—To create a weight composition ratio of 2-98%        SMG-PEEK, 98 g of PEEK and 2 g of SMG were added to the mixer.        After 25 minutes, of mixing time, the port was opened to allow        G-PMC to flow out as extrudate and 5.1 g were extruded out until        no more material was able to flow.

Characterization of Graphene-Reinforced PEEK

The samples used for characterization appear in Table 3, as follows:

TABLE 3 Samples Used for Characterization Batch Mixer Sample Description(Capacity) Graph Color PEEK_3 Control mixed for 3 minutes 10 g GreenPEEK_90 Control mixed for 90 minutes 100 g  Purple SMG- Components mixedfor 3 10 g Orange PEEK_3 minutes SMG- Components mixed for 30 10 g BluePEEK_30 minutes SMG- Components mixed for 90 10 g Red PEEK_90 minutes

Morphology

The morphology of the xG-PMC was examined using a Zeiss Sigma FieldEmission Scanning Electron Microscope (“FESEM”) with Oxford EDS. Anaccelerating voltage of 3 kV and working distance of approximately 8.5mm was used during viewing. Prior to viewing, specimens were notched,cryogenically fractured to produce a flat fracture surface, placed undervacuum for at least 24 hours, gold coated, and stored under vacuum. Asillustrated in FIG. 3, the morphology of SMG-PEEK_90 is shown in (a) 10μm scale and 1,000 magnification (b) 10 μm scale and 5,000magnification, (c) 1 μm scale and 10,000 magnification, and (d) 1 μmscale and 50,000 magnification.

Thermal Analysis

The thermal properties of the samples were characterized using a TAInstruments Q1000 Differential Scanning calorimeter (DSC). Each samplewas subject to a heat/cool/heat cycle from 0-400° C. at 10° C./min. Theglass transition temperature (Tg) and melting temperature (Tm) for theinitial heat scan are illustrated in FIG. 3. The Tg increases from 152°C. for PEEK_3 to 154 for SMG-PEEK_90, however, this increase is notsignificant. The Tm is consistent for samples PEEK_3, SMG-PEEK_3, andSMG-PEEK_30 at almost 338° C. but decreases significantly to 331.7° C.for SMG-PEEK_90. The delta H is similar for samples PEEK_3, SMG-PEEK_3,and SMG-PEEK_30, and varies between the initial, cool, and reheat scans,and ranges between 116-140 J/g. However, the delta H for SMG-PEEK_90 ismuch lower and consistent at approximately 100 J/g for the initial,cool, and reheat scans. The observable difference in the heat of fusionof PEEK for the SMG-PEEK_90 sample, as compared with the other samples,indicates a major difference in the morphology. Furthermore, theconstant heat of fusion between the initial, cool, and reheat scans ofthe SMG-PEEK_90 sample supports the existence of cross links between thegraphene and PEEK matrix.

Parallel Plate Rheology

A frequency sweep from 100-0.01 Hz at 1.0% strain and at a temperatureof 360° C. was performed using a TA Instruments AR 2000 in parallelplate mode. Samples SMG-PEEK_30, SMG-PEEK_3, and PEEK_3 were tested. TheG′ and G″ and the tan delta for samples SMG-PEEK_30, SMG-PEEK_3, andPEEK_3 were recorded. Tan delta is equal to the G″/G′. This rheologydata provides information regarding the morphology of the sample,according to Table 4, as shown below. The sol/gel transition point, or“gel point”, of a thermoset resin occurs when tan delta=1, or ratherwhen G′=G″. For samples SMG-PEEK_3 and PEEK_3, the G″ is greater thanthe G′, indicating liquid-like behavior. Contrastingly for sampleSMG-PEEK_30, the G′ is greater than G″, indicating more elastic-like orsolid-like behavior. Furthermore, tan delta is less than 1 and remainsnearly constant across the entire frequency range for SMG-PEEK_30,indicating that SMG-PEEK_30 has undergone some degree of cross-linking.

TABLE 4 Rheology data and the sol/gel transition point Shear and StateMorphology Tan δ Loss Moduli Sample Behavior Liquid “Sol State” >1 G″ >G′ PEEK_3 state SMG-PEEK_3 Gel point Cross linking begins =1 G′ = G″ GelState Solid State <1 G′ > G″ SMG-PEEK_30 Sample contains cross-links

Dissolution

Lightly gelled thermosetting resins when placed in solvents swellthrough imbibition to a degree depending on the solvent and thestructure of the polymer. The original shape is preserved, and theswollen gel exhibits elastic rather than plastic properties.Cross-linking in thermoplastic polymers is commonly accomplished by 1)peroxides, 2) a grafted silane process cross-linked by water, 3)electron beam radiation, and 4) UV light.

Example 6C. Cross-Linked Graphene-Reinforced PEEK (G-PEEK)

In this example, cross-linking was induced between SMG and PEEK during amechanical exfoliation process due to the cleavage of graphene flakesthat results in dangling free radicals. To confirm the presence ofcross-linking in the SMG-PEEK XG-PMC, a dissolution method was used byplacing neat PEEK, PEEK_3, PEEK_90, SMG-PEEK_3, SMG-PEEK_30, andSMG-PEEK_90 samples in sulfuric acid, according to the following steps.

-   -   A 10 mg specimen from each sample was prepared;    -   Each specimen was placed in a test tube with 20 mL of 95-98% w/w        sulfuric acid (A300S500 Fisher Scientific);    -   The solution was shaken for 5 minutes;    -   Each test tube was capped with TEFLON® tape to form a seal;    -   Photographs of each sample were taken at times 0, 24, 48, and 72        hours.

Upon visual observation, the PEEK samples all dissolve within thesulfuric acid before 24 hours, and the SMG-PEEK_90 sample is the onlyone that remains in the sulfuric acid after 72 hours. The SMG-PEEK_90sample was cross-linked and swelled when placed in the solvent similarto a thermoset resin. The SMG-PEEK_30 sample remained in the sulfuricacid after 24 hours but dissolved before 48 hours. SMG-PEEK_30 requiredfurther testing to determine if cross-linking was induced, since theother data suggests that SMG-PEEK_30 was cross-linked.

Example 7. Graphene-Reinforced Polyamide-6,6 (G-PA66) MaterialsProcessing

Using a continuous process, well-crystallized graphite (45 mesh) from amined source was added at 35 wt % to PA66 and exfoliated in moltenpolymer at 277° C. to form G-PA66. Prior to melt-processing, graphiteand PA66 were dried overnight at 300° C. and 85° C., respectively; neatPA66 was also processed as a control. Tensile and impact bars wereproduced, according to ASTM D 638 Type I and ASTM D 256 specifications,respectively. ASTM D 638 Type V tensile bars of G-PA66 were alsoprocessed using a similar batch processing method, which impartsenhanced exfoliation and mixing.

Materials Characterization

Microstructures of PA66 and G-PA66 samples were examined using a ZeissSigma Field Emission Scanning Electron Microscope (FESEM). Specimenswere prepared by cryogenic fracture; one fracture surface was sputtercoated with gold (5 nm), and the opposing surface was uncoated.

Tensile properties were evaluated using a MTS QTest/25 Universal testingmachine, according to ASTM D638 for Type I and Type V specimens. IzodImpact properties were determined on notched specimens, using an InstronDynatup POE 2000 Impact Testing Machine with average impact velocity of3.47 m/sec, according to ASTM D256; the same pendulum weight was usedfor each specimen. In all cases, a minimum of 10 specimens were impacttested per sample.

Morphology

The morphology of G-PA66 Type I specimens is shown in FIG. 14 atdifferent scales (i.e. magnification). A uniform distribution ofgraphene flakes within the PA66 is evident in (a) and (b) and goodadhesion between graphene flakes and PA66 matrix are indicated in (c)and (d). A transparent graphene flake is shown in (e). The morphology ofG-PA66 Type V specimens is shown in FIG. 15 at different scales (i.e.magnification). Once again, good distribution and adhesion of grapheneflakes within the PA66 are evident in (a) and (b); examples oftransparent flakes are shown in (d)-(f).

Tensile Properties

The modulus, stress and % strain at yield, and stress and % strain atbreak for Type I PA66 and G-PA66 tensile specimens, prepared using acontinuous mixing process, are shown in FIG. 16. The modulus increasessignificantly with the addition of graphene to PA66 from 3.2 GPa to 7.9GPa. The stress at yield and stress at break both decrease slightly. The% strain at yield and % strain at break decrease significantly.

Similar results for Type V PA66 and G-PA66 tensile specimens, preparedusing a batch mixing process, are shown in FIG. 17. The modulusincreases significantly with the addition of graphene to PA66 from 2.4GPa to 13.0 GPa. The stress at yield and stress at break both increasewith the addition of graphene from about 50 MPa to over 60 MPa. The %strain at yield and % strain at break decrease significantly.

Impact Properties

The notched Izod impact results for PA66 and G-PA66, prepared using acontinuous process, are shown in FIG. 18. All specimens underwentcomplete fracture upon impact.

Comparison with Graphene-Reinforced PEEK

Using the same continuous melt-processing method, 35% graphite wasexfoliated within molten polyetheretherketone (PEEK) to form a G-PEEKcomposite. Neat PEEK was processed as the control. The graphite and PEEKwere dried overnight at 300° C. and 160° C., respectively, and processedbetween 370-390° C. Tensile and impact bars were produced, according toASTM D 638 Type I and ASTM D 256 specifications, respectively. Tensileand impact properties were characterized for PEEK and G-PEEK, accordingto ASTM D 638 and ASTM D 256, respectively.

Microstructures of G-PEEK specimens, after cryogenic fracture, are shownat different scales (i.e. increasing magnification) in FIG. 19. Gooddistribution and orientation of the graphene in PEEK is evident. Thecorresponding tensile modulus, stress and % strain at yield, and stressand % strain at break for PEEK and G-PEEK are shown in FIG. 20. With theaddition of graphene to PEEK, the modulus increases significantly from3.99 GPa to 18.54 GPa, and the stress at yield increases from 87 to 101MPa. The stress at break remains constant at 101 MPa. The % strain atyield decreases slightly, and % strain at break decreases significantly,as is common with fiber-reinforced composites. Notched Izod impactresults for PEEK and G-PEEK are shown in FIG. 21. All specimensfractured completely. With the addition of graphene to PEEK, the impactresist-ance decreases slightly from 385 Jim to 331 Jim.

Example 8. Graphene-Reinforced Polystyrene (G-PS)

High purity flake graphite and PS were separately dried in an oven for10-12 hours at 300° C. and 70° C., respectively, to remove any absorbedwater prior to processing. The components were dry-blended in a 35:65 wt% ratio of graphite:PS followed by melt-processing at 216° C.,exfoliating the graphite within the molten polymer as above to provide agraphene nano-flake reinforced polymer composite. ASTM Type 1 specimenswere produced from the G-PS composite. Using the same method, neat PSspecimens were produced for comparison. The mechanical properties inflexure were characterized according to ASTM D 790. The flexural modulusof PS and G-PS is shown in FIG. 22, and reveals a significant increasein modulus for graphene nano-flake reinforced polystyrene (G-PS), ascompared with neat PS.

The foregoing examples and description of the preferred embodimentsshould be taken as illustrating, rather than as limiting the presentinvention as defined by the claims. As will be readily appreciated,numerous variations and combinations of the features set forth above canbe utilized without departing from the present invention as set forth inthe claims. Such variations are not regarded as a departure from thespirit and scope of the invention, and all such variations are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A graphene-reinforced polymer matrix compositecomprising an essentially uniform distribution in a thermoplasticpolymer matrix of up to about 50% of total composite weight of particlesselected from the group consisting of graphite microparticles,single-layer graphene nanoparticles, multi-layer graphene nanoparticles,and combinations of two or more thereof wherein: said particles comprisesingle- and/or multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction; and said thermoplasticpolymer is selected from the group consisting of polyamides,polystyrenes, polyphenylene sulfides, high-density polyethylenes, ABSpolymers, polyacrylonitriles, polylactic acids, polyglycolic acids,polylactic-glycolic acid copolymers (PLGA) and mixtures of two or morethereof.
 2. The graphene-reinforced polymer matrix composite of claim 1,wherein said thermoplastic polymer comprises a polyamide, a polystyrene,a polyphenylene sulfide or a high-density polyethylene.
 3. Thegraphene-reinforced polymer matrix composite of claim 2, wherein saidpolyamide is selected from the group consisting of aliphatic polyamides,semi-aromatic polyamides, aromatic polyamides and combinations of two ormore thereof.
 4. The graphene-reinforced polymer matrix composite ofclaim 3, wherein said polyamide is an aliphatic polyamide selected fromthe group consisting of polyamide-6,6; polyamide-6,9; polyamide-6,10;polyamide-6,12; polyamide-4,6; polyamide-6 (nylon-6); polyamide-11(nylon-11) and polyamide-12 (nylon-12).
 5. The graphene-reinforcedpolymer matrix composite of claim 4, wherein said aliphatic polyamide ispolyamide-6,6.
 6. The graphene-reinforced polymer matrix composite ofclaim 3, wherein said polyamide is a semi-aromatic polyamide.
 7. Thegraphene-reinforced polymer matrix composite of claim 6, wherein saidsemi-aromatic polyamide is a polyphthalamide.
 8. The graphene-reinforcedpolymer matrix composite of claim 3, wherein said polyamide is anaromatic polyamide.
 9. The graphene-reinforced polymer matrix compositeof claim 8, wherein said aromatic polyamide is a para-aramid.
 10. Thegraphene-reinforced polymer matrix composite of claim 1, wherein thegraphite may be doped with other elements to modify a surface chemistryof the exfoliated graphene nanoparticles.
 11. The graphene-reinforcedpolymer matrix composite of claim 1, wherein the graphite is expandedgraphite.
 12. The graphene-reinforced polymer matrix composite of claim1, wherein a surface chemistry or nano structure of the dispersedgraphite may be modified to bond with the polymer matrix to increasestrength and stiffness of the graphene-reinforced composite.
 13. Thegraphene-reinforced polymer matrix composite of claim 1, whereindirectional alignment of the graphene nanoparticles is used to obtainone-, two- or three-dimensional reinforcement of the polymer matrixphase.
 14. The graphene-reinforced polymer matrix composite of claim 1,comprising polymer chains inter-molecularly cross-linked by single- ormulti-layer graphene sheets having carbon atoms with reactive bondingsites on the edges of said sheets.
 15. An automotive, aircraft,watercraft or aerospace part formed from the graphene-reinforced polymermatrix composite of claim
 1. 16. The part of claim 15, wherein said partis an engine part.
 17. The graphene-reinforced polymer matrix compositeof claim 1, wherein said composite is prepared by a method comprisingthe steps of: (a) distributing graphite microparticles into a moltenthermoplastic polymer phase comprising one or more of said matrixpolymers, wherein at least 50% of the graphite in the graphitemicroparticles consists of multilayer graphite crystals between 1.0 and1000 microns thick along a c-axis direction; and (b) applying asuccession of shear strain events to said molten polymer phase so thatthe shear stress within said molten polymer phase is equal to or greaterthan the Interlayer Shear Strength (ISS) of said graphite microparticlesand said molten polymer phase exfoliates said graphite successively witheach event until said graphite is at least partially exfoliated to forma distribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.
 18. The graphene-reinforced polymer matrix composite of claim17, wherein the graphite particles are prepared by crushing and grindinga graphite-containing mineral to millimeter-sized dimensions, reducingthe millimeter-sized particles to micron-sized dimensions, andextracting micron-sized graphite particles from the graphite-containingmineral.
 19. The graphene-reinforced polymer matrix composite of claim17, wherein said graphite particles are distributed into said moltenpolymer phase using a single screw extruder with axial flutedextensional mixing elements or spiral fluted extensional mixingelements.
 20. The graphene-reinforced polymer matrix composite of claim19, wherein the graphite-containing molten polymer phase is subjected torepeated extrusion to induce exfoliation of the graphitic material andform said essentially uniform dispersion of said single- and multi-layergraphene nanoparticles in said thermoplastic polymer matrix.